Airfoil, Airfoil, Airfoil

The single biggest consideration and the one I spent the most time on was the Airfoil.  The airfoil selection can make or break a design.  There are tons of published airfoils to choose from but there are three basic types: lifting, symmetrical and semi-symmetrical.  Each type of airfoil is designed for a specific requirement.

Aerodynamic design is a game of compromise, you cant get something for nothing.  Every design decision is a balance.  I wanted to see what others designers were using and why.  A symmetrical airfoil is a good choice for a high performance wing such as a race wing.  These types of wings are usually thin and flown fast.  There are several advantages to a symmetrical airfoil.  Symmetrical airfoils are a non-lifting airfoil, meaning their shape doesn't generate lift.  They must generate lift by deflecting the airflow down off of the bottom of the wing.  The generation of lift causes induced drag.  Because the symmetrical airfoil doesn't generate a lot of lift it also doesn't create a lot of drag making it a great choice for performance.  Also, a symmetrical airfoil doesn't have a negative pitching moment (more on that later) and it fly the same inverted as it does right side up.  The disadvantages of a symmetrical airfoil (remember the balance?) is that they don't generate a lot of lift.  This means they must be moving fast to stay in the air.  Symmetrical airfoils have a high stall speed and can't carry as much payload.  As they say "if they ain't hauling they're falling".

So clearly a symmetrical airfoil would not be the proper choice for my wing given the cruising speed and flight time requirements.  A thicker lifting airfoil would be a more appropriate choice for this design.  I looked to Sweepwings.com for inspiration.  Ruben, the owner has been extremely  helpful and just a hell of a nice guy.  I listened to a couple of podcasts that he was on which actually inspired me to start this whole process.  He also has his own podcast now, FPVRaw.  Ruben's wings use thicker, lifting airfoils.  This allows them to carry heavier payloads and to fly at lower cruising speeds. This was the direction I wanted to go so I started to take a closer look at his planes.  As with anything in aerodynamics there are trade-offs with this type of airfoil. Lifting airfoils, as the name implies, generate lift and the act of generating lift also generates drag.  The trick is to find an airfoil that has an acceptable lift to drag ratio. This, it turned out, was a very interesting journey.  There are several online libraries of airfoils available and most include the point files needed to analyze the airfoil.  I liked AirfoilTools in particular.

I started by doing research on airfoils for tailless aircraft.  Flying wings have their own special set of requirements.  The absence of a tail forces the wing to do the work of both the vertical and horizontal stabilizers or elevator and rudder.  What wings lack in parts they make up for in nuance. One of the primary issues is pitching moment.  I'm going to try to explain what the pitching moment is and its implication for a flying wing.  This may get long and boring so you can just skip down a couple of sections if you want, you won't hurt my feelings.

How it works

There are a lot of great explanations of this out there but this is how I see it.  There are forces acting on the wing in 4 directions.  The two we will be taking about now are lift and weight.  The wing, and aircraft, has a mass and is being drawn to the center of the earth by gravity.  This downward force that is the enemy of all pilots has a balance point in the horizontal plane.  This is the Center of Gravity or CG (shown as mg above).  This is the force we are trying to counteract with an opposite force, Lift.  Lift is the vertical force generated by the wing and has a balance point called the Center of Pressure, Cp (shown as L above).  The CG is constant during flight unless something has broken loose and is rolling abound inside your plane.  The Cp location can actually move forward and back based on the angle of attack and your specific airfoil.  It is important to find an airfoil that has limited Cp movement withing your flight envelope.  For stability reasons we want a flying wing to be nose heavy.  This will give us positive control of the wing.  There is a saying, " a nose heavy plane flies poorly, a tail-heavy plane flies once".  Because the CG is in front of the Cp there is a tendency for the wing to pitch forward, rotating around the CG.  This is called the pitching moment.

Most airplanes counteract this pitching moment by adding downward force with the tail, and you guessed it, we don't have a tail.  The wing itself must compensate for this pitching moment.  This can be done several ways.  First of all, choose an airfoil with a low pitching moment.  Secondly we will need some way to generate a positive pitching moment. This means we need to have some amount of reflex in the wing.  This can be generated by the elevons being angled up slightly or by selecting a reflex airfoil, like the one shown above.  The trailing edge of the airfoil curves up forcing the back of the wing down.  Everything is a compromise and reflex = drag.  This is why flying wings will never be as efficient as a conventional airplane.  A conventional plane can use far less downward force in the tail because it has a much longer moment arm or lever because distance to the elevator is mush greater.